Banff Springs Hotel. It won't be cheap but I think it will be worth it. I'm sure a few of the restos on Banff Ave will also be open. You can also look into Rimrock hotel in banff too.
The Banff Springs hotel has about 7 restos inside so they may even be offeriing different XMAS dinner options.
As for Canmore options, well since most of their hotels are condo type projects they just don't have restos attached that are worth checking out. Merry Christmas and happy dining!